One type of power converters is designed to perform zero-voltage switching control, referred to simply as ZVS control. The ZVS control is to perform switching of a switch or a switching element while a voltage across the switching element is zero. Power converters, which are capable of performing the ZVS control, aim to reduce switching loss of their switching elements to have higher efficiency accordingly. An example of these power converters is disclosed as a DC-DC converter in Japanese Patent Application No. 2004-129393, which is referred to as patent document 1.
The DC-DC converter disclosed in patent document 1 includes first and second main switches connected in series between input terminals of the DC-DC converter, and a smoothing inductor connected between an output terminal of the DC-DC converter and the connection point between the first and second main switches. The DC-DC converter also includes an auxiliary resonance circuit, which is comprised of a resonance inductor and an auxiliary switch, connected between the output terminal and the connection point between the first and second main switches. The DC-DC converter further includes resonance capacitors connected in parallel to the respective first and second main switches.
The DC-DC converter measures an auxiliary current flowing through the auxiliary resonance circuit using a current sensor.
When the measured auxiliary current satisfies a predetermined ZVS condition, the DC-DC converter turns off the second main switch, and turns on the auxiliary switch within the period from turn-off of the second main switch to turn-on of the first main switch. This results in electrical energy being supplied from the output terminal to the resonance inductor. This causes the resonance inductor and the resonance capacitors to resonate with each other.
After lapse of predetermined dead time since the turn-off of the second main switch, the DC-DC converter turns on the first main switch while the voltage across the first main switch is kept zero. This therefore results in reliable ZVS control of the first main switch.
Note that the predetermined ZVS condition is that the measured auxiliary current is equal to or higher than a value determined based on the input and output voltages, the capacitances of the respective resonance capacitors, and the inductance of the resonance inductor.